.\"	$NetBSD: ptrace.2,v 1.37 2015/07/02 03:50:21 christos Exp $
.\"
.\" This file is in the public domain.
.Dd July 1, 2015
.Dt PTRACE 2
.Os
.Sh NAME
.Nm ptrace
.Nd process tracing and debugging
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/ptrace.h
.Ft int
.Fn ptrace "int request" "pid_t pid" "void *addr" "int data"
.Sh DESCRIPTION
.Fn ptrace
provides tracing and debugging facilities.
It allows one process (the
.Em tracing
process) to control another (the
.Em traced
process).
Most of the time, the traced process runs normally, but when
it receives a signal
.Po
see
.Xr sigaction 2
.Pc ,
it stops.
The tracing process is expected to notice this via
.Xr wait 2
or the delivery of a
.Dv SIGCHLD
signal, examine the state of the stopped process, and cause it to
terminate or continue as appropriate.
.Fn ptrace
is the mechanism by which all this happens.
.Pp
The
.Fa request
argument specifies what operation is being performed; the meaning of
the rest of the arguments depends on the operation, but except for one
special case noted below, all
.Fn ptrace
calls are made by the tracing process, and the
.Fa pid
argument specifies the process ID of the traced process.
.Fa request
can be:
.Bl -tag -width 12n
.It Dv PT_TRACE_ME
This request is the only one used by the traced process; it declares
that the process expects to be traced by its parent.
All the other arguments are ignored.
(If the parent process does not expect to trace
the child, it will probably be rather confused by the results; once the
traced process stops, it cannot be made to continue except via
.Fn ptrace . )
When a process has used this request and calls
.Xr execve 2
or any of the routines built on it
.Po
such as
.Xr execv 3
.Pc ,
it will stop before executing the first instruction of the new image.
Also, any setuid or setgid bits on the executable being executed will
be ignored.
.It Dv PT_READ_I , Dv PT_READ_D
These requests read a single
.Li int
of data from the traced process' address space.
Traditionally,
.Fn ptrace
has allowed for machines with distinct address spaces for instruction
and data, which is why there are two requests: conceptually,
.Dv PT_READ_I
reads from the instruction space and
.Dv PT_READ_D
reads from the data space.
In the current
.Nx
implementation, these
two requests are completely identical.
The
.Fa addr
argument specifies the address (in the traced process' virtual address
space) at which the read is to be done.
This address does not have to meet any alignment constraints.
The value read is returned as the return value from
.Eo \&
.Fn ptrace
.Ec .
.It Dv PT_WRITE_I , Dv PT_WRITE_D
These requests parallel
.Dv PT_READ_I
and
.Dv PT_READ_D ,
except that they write rather than read.
The
.Fa data
argument supplies the value to be written.
.\" .It Dv PT_READ_U
.\" This request reads an
.\" .Li int
.\" from the traced process' user structure.
.\" The
.\" .Fa addr
.\" argument specifies the location of the int relative to the base of the
.\" user structure; it will usually be an integer value cast to
.\" .Li void *
.\" either explicitly or via the presence of a prototype for
.\" .Eo \&
.\" .Fn ptrace
.\" .Ec .
.\" Unlike
.\" .Dv PT_READ_I
.\" and
.\" .Dv PT_READ_D ,
.\" .Fa addr
.\" must be aligned on an
.\" .Li int
.\" boundary.
.\" The value read is returned as the return value from
.\" .Eo \&
.\" .Fn ptrace
.\" .Ec .
.\" .It Dv PT_WRITE_U
.\" This request writes an
.\" .Li int
.\" into the traced process' user structure.
.\" .Fa addr
.\" specifies the offset, just as for
.\" .Dv PT_READ_U ,
.\" and
.\" .Fa data
.\" specifies the value to be written, just as for
.\" .Dv PT_WRITE_I
.\" and
.\" .Dv PT_WRITE_D .
.It Dv PT_CONTINUE
The traced process continues execution.
.Fa addr
is an address specifying the place where execution is to be resumed (a
new value for the program counter), or
.Li (void *)1
to indicate that execution is to pick up where it left off.
.Fa data
provides a signal number to be delivered to the traced process as it
resumes execution, or 0 if no signal is to be sent.
If a negative value is supplied, that is the negative of the LWP
ID of the thread to be resumed, and only that thread executes.
.It Dv PT_KILL
The traced process terminates, as if
.Dv PT_CONTINUE
had been used with
.Dv SIGKILL
given as the signal to be delivered.
.It Dv PT_ATTACH
This request allows a process to gain control of an otherwise unrelated
process and begin tracing it.
It does not need any cooperation from the to-be-traced process.
In this case,
.Fa pid
specifies the process ID of the to-be-traced process, and the other two
arguments are ignored.
This request requires that the target process
must have the same real UID as the tracing process, and that it must
not be executing a setuid or setgid executable.
(If the tracing process is running as root,
these restrictions do not apply.)
The tracing process will see the newly-traced process stop and may then
control it as if it had been traced all along.
.Pp
Three other restrictions apply to all tracing processes, even those
running as root.
First, no process may trace a system process.
Second, no process may trace the process running
.Xr init 8 .
Third, if a process has its root directory set with
.Xr chroot 2 ,
it may not trace another process unless that process's root directory
is at or below the tracing process's root.
.It Dv PT_DETACH
This request is like PT_CONTINUE, except that after it
succeeds, the traced process is no longer traced and continues
execution normally.
.It Dv PT_IO
This request is a more general interface that can be used instead of
.Dv PT_READ_D ,
.Dv PT_WRITE_D ,
.Dv PT_READ_I ,
and
.Dv PT_WRITE_I .
The I/O request is encoded in a
.Dq Li "struct ptrace_io_desc"
defined as:
.Bd -literal -offset indent
struct ptrace_io_desc {
	int	piod_op;
	void	*piod_offs;
	void	*piod_addr;
	size_t	piod_len;
};
.Ed
.Pp
where
.Fa piod_offs
is the offset within the traced process where the I/O operation should
take place,
.Fa piod_addr
is the buffer in the tracing process, and
.Fa piod_len
is the length of the I/O request.
The
.Fa piod_op
field specifies which type of I/O operation to perform.
Possible values are:
.Pp
.Bl -tag -width 18n -offset indent -compact
.It Dv PIOD_READ_D
.It Dv PIOD_WRITE_D
.It Dv PIOD_READ_I
.It Dv PIOD_WRITE_I
.It Dv PIOD_READ_AUXV
.El
.Pp
See the description of
.Dv PT_READ_I
for the difference between I and D spaces.
The
.Dv PIOD_READ_AUXV
operation can be used to read from the ELF auxiliary vector.
A pointer to the I/O descriptor is passed in the
.Fa addr
argument to
.Fn ptrace .
On return, the
.Fa piod_len
field in the I/O descriptor will be updated with the actual number of
bytes transferred.
If the requested I/O could not be successfully performed,
.Fn ptrace
will return
.Li \-1
and set
.Va errno .
.It Dv PT_DUMPCORE
Makes the process specified in the
.Fa pid
pid generate a core dump.
The
.Fa addr
argument should contain the name of the core file to be generated
and the
.Fa data
argument should contain the length of the core filename.
This
.Nm
call currently does not stop the child process so it can generate
inconsistent data.
.It Dv PT_LWPINFO
Returns information about a thread from the list of threads for the
process specified in the
.Fa pid
argument.
The
.Fa addr
argument should contain a
.Dq Li "struct ptrace_lwpinfo"
defined as:
.Bd -literal -offset indent
struct ptrace_lwpinfo {
	lwpid_t pl_lwpid;
	int pl_event;
};
.Ed
.Pp
where
.Fa pl_lwpid
contains a thread LWP ID.
Information is returned for the thread following the one with the
specified ID in the process thread list, or for the first thread
if
.Fa pl_lwpid
is 0.
Upon return
.Fa pl_lwpid
contains the LWP ID of the thread that was found, or 0 if there is
no thread after the one whose LWP ID was supplied in the call.
.Fa pl_event
contains the event that stopped the thread.
Possible values are:
.Pp
.Bl -tag -width 30n -offset indent -compact
.It Dv PL_EVENT_NONE
.It Dv PL_EVENT_SIGNAL
.El
.Pp
The
.Fa data
argument should contain
.Dq Li "sizeof(struct ptrace_lwpinfo)" .
.It Dv PT_SYSCALL
Stops a process before and after executing each system call.
.It Dv PT_SYSCALLEMU
Intercept and ignore a system call before it has been executed, for use with
.Dv PT_SYSCALL .
.El
.Pp
Additionally, the following requests exist but are
not available on all machine architectures.
The file
.In machine/ptrace.h
lists which requests exist on a given machine.
.Bl -tag -width 12n
.It Dv PT_STEP
Execution continues as in request PT_CONTINUE; however
as soon as possible after execution of at least one
instruction, execution stops again.
If the
.Fa data
argument is greater than 0, it contains the LWP ID of the thread to be
stepped, and any other threads are continued.
If the
.Fa data
argument is less than zero, it contains the negative of the LWP ID of
the thread to be stepped, and only that thread executes.
.It Dv PT_GETREGS
This request reads the traced process' machine registers into the
.Dq Li "struct reg"
(defined in
.In machine/reg.h )
pointed to by
.Fa addr .
The
.Fa data
argument contains the LWP ID of the thread whose registers are to
be read.
If zero is supplied, the first thread of the process is read.
.It Dv PT_SETREGS
This request is the converse of
.Dv PT_GETREGS ;
it loads the traced process' machine registers from the
.Dq Li "struct reg"
(defined in
.In machine/reg.h )
pointed to by
.Fa addr .
The
.Fa data
argument contains the LWP ID of the thread whose registers are to
be written.
If zero is supplied, the first thread of the process is written.
.It Dv PT_GETFPREGS
This request reads the traced process' floating-point registers into
the
.Dq Li "struct fpreg"
(defined in
.In machine/reg.h )
pointed to by
.Fa addr .
The
.Fa data
argument contains the LWP ID of the thread whose registers are to
be read.
If zero is supplied, the first thread of the process is read.
.It Dv PT_SETFPREGS
This request is the converse of
.Dv PT_GETFPREGS ;
it loads the traced process' floating-point registers from the
.Dq Li "struct fpreg"
(defined in
.In machine/reg.h )
pointed to by
.Fa addr .
The
.Fa data
argument contains the LWP ID of the thread whose registers are to
be written.
If zero is supplied, the first thread of the process is written.
.\" .It Dv PT_SYSCALL
.\" This request is like
.\" .Dv PT_CONTINUE
.\" except that the process will stop next time it executes any system
.\" call.
.\" Information about the system call can be examined with
.\" .Dv PT_READ_U
.\" and potentially modified with
.\" .Dv PT_WRITE_U
.\" through the
.\" .Li u_kproc.kp_proc.p_md
.\" element of the user structure (see below).
.\" If the process is continued
.\" with another
.\" .Dv PT_SYSCALL
.\" request, it will stop again on exit from the syscall, at which point
.\" the return values can be examined and potentially changed.
.\" The
.\" .Li u_kproc.kp_proc.p_md
.\" element is of type
.\" .Dq Li "struct mdproc" ,
.\" which should be declared by including
.\" .In sys/param.h ,
.\" .In sys/user.h ,
.\" and
.\" .In machine/proc.h ,
.\" and contains the following fields (among others):
.\" .Bl -item -compact -offset indent
.\" .It
.\" .Li syscall_num
.\" .It
.\" .Li syscall_nargs
.\" .It
.\" .Li syscall_args[8]
.\" .It
.\" .Li syscall_err
.\" .It
.\" .Li syscall_rv[2]
.\" .El
.\" When a process stops on entry to a syscall,
.\" .Li syscall_num
.\" holds the number of the syscall,
.\" .Li syscall_nargs
.\" holds the number of arguments it expects, and
.\" .Li syscall_args
.\" holds the arguments themselves.
.\" (Only the first
.\" .Li syscall_nargs
.\" elements of
.\" .Li syscall_args
.\" are guaranteed to be useful.)
.\" When a process stops on exit from a syscall,
.\" .Li syscall_num
.\" is
.\" .Eo \&
.\" .Li \-1
.\" .Ec ,
.\" .Li syscall_err
.\" holds the error number
.\" .Po
.\" see
.\" .Xr errno 2
.\" .Pc ,
.\" or 0 if no error occurred, and
.\" .Li syscall_rv
.\" holds the return values.
.\" (If the syscall returns only one value, only
.\" .Li syscall_rv[0]
.\" is useful.)
.\" The tracing process can modify any of these with
.\" .Dv PT_WRITE_U ;
.\" only some modifications are useful.
.\" .Pp
.\" On entry to a syscall,
.\" .Li syscall_num
.\" can be changed, and the syscall actually performed will correspond to
.\" the new number (it is the responsibility of the tracing process to fill
.\" in
.\" .Li syscall_args
.\" appropriately for the new call, but there is no need to modify
.\" .Eo \&
.\" .Li syscall_nargs
.\" .Ec ).
.\" If the new syscall number is 0, no syscall is actually performed;
.\" instead,
.\" .Li syscall_err
.\" and
.\" .Li syscall_rv
.\" are passed back to the traced process directly (and therefore should be
.\" filled in).
.\" If the syscall number is otherwise out of range, a dummy
.\" syscall which simply produces an
.\" .Er ENOSYS
.\" error is effectively performed.
.\" .Pp
.\" On exit from a syscall, only
.\" .Li syscall_err
.\" and
.\" .Li syscall_rv
.\" can usefully be changed; they are set to the values returned by the
.\" syscall and will be passed back to the traced process by the normal
.\" syscall return mechanism.
.It Dv PT_DUMPCORE
Cause the traced process to dump core.
If the
.Fa addr
argument is not
.Dv NULL
it is taken to be the pathname of the core file to be generated and the
.Fa data
argument should contain the length of the pathname.
The pathname may contain
.Dv %
patterns that are expanded as described in
.Xr sysctl 8 .
If the
.Fa data
argument is
.Dv NULL ,
the default core file path generation rules are followed.
.El
.Sh ERRORS
Some requests can cause
.Fn ptrace
to return
.Li \-1
as a non-error value; to disambiguate,
.Va errno
can be set to 0 before the call and checked afterwards.
The possible errors are:
.Bl -tag -width "[EINVAL]"
.It Bq Er EAGAIN
Process is currently exec'ing and cannot be traced.
.It Bq Er EBUSY
.Bl -bullet -compact
.It
.Dv PT_ATTACH
was attempted on a process that was already being traced.
.It
A request attempted to manipulate a process that was being traced by
some process other than the one making the request.
.It
A request (other than
.Dv PT_ATTACH )
specified a process that wasn't stopped.
.El
.It Bq Er EINVAL
.Bl -bullet -compact
.It
A process attempted to use
.Dv PT_ATTACH
on itself.
.It
The
.Fa request
was not a legal request on this machine architecture.
.\" .It
.\" The
.\" .Fa addr
.\" to
.\" .Dv PT_READ_U
.\" or
.\" .Dv PT_WRITE_U
.\" was not
.\" .Li int Ns \&-aligned.
.It
The signal number (in
.Fa data )
to
.Dv PT_CONTINUE
.\" or
.\" .Dv PT_SYSCALL
was neither 0 nor a legal signal number.
.It
.Dv PT_GETREGS ,
.Dv PT_SETREGS ,
.Dv PT_GETFPREGS ,
or
.Dv PT_SETFPREGS
was attempted on a process with no valid register set.
(This is normally true only of system processes.)
.El
.It Bq Er EPERM
.Bl -bullet -compact
.It
A request (other than
.Dv PT_ATTACH )
attempted to manipulate a process that wasn't being traced at all.
.It
An attempt was made to use
.Dv PT_ATTACH
on a process in violation of the requirements listed under
.Dv PT_ATTACH
above.
.El
.It Bq Er ESRCH
No process having the specified process ID exists.
.El
.Sh SEE ALSO
.Xr sigaction 2 ,
.Xr signal 7
.Sh BUGS
On the SPARC, the PC is set to the provided PC value for
.Dv PT_CONTINUE
and similar calls,
but the NPC is set willy-nilly to 4 greater than the PC value.
Using
.Dv PT_GETREGS
and
.Dv PT_SETREGS
to modify the PC, passing
.Li (void *)1
to
.Eo \&
.Fn ptrace
.Ec ,
should be able to sidestep this.
.\" .Pp
.\" When using
.\" .Dv PT_SYSCALL ,
.\" there is no easy way to tell whether the traced process stopped because
.\" it made a syscall or because a signal was sent at a moment that it just
.\" happened to have valid-looking garbage in its
.\" .Dq Li "struct mdproc" .
